In relatively large tractors and/or other agricultural implements, a multiple ratio gear drive assembly is usually provided as an integral part of a power train assembly. The gear drive assembly includes a plurality of hydraulically actuated power-shift clutch assemblies serially arranged within a limited space to allow the gear drive assembly to rapidly shift from one gear ratio to another. The clutch assemblies are sequentially operated to connect/disconnect a rotating input member and rotational output members until a desired speed for the tractor or the like is accomplished.
A typical hydraulic clutch assembly includes a plurality of interleaved clutch plates alternate ones of which are spline connected to and rotate with a rotating input member and the remainder of which are spline connected to and rotate with a rotational output member. These interleaved clutch plates transfer rotational motion and torque between the rotating members as a function of the axial compression of such plates by a clutch actuator.
The clutch actuator includes an annular, fluid responsive piston which is operative to apply a compressive force to the interleaved plates. To economize on space, such hydraulic clutch assemblies are normally designed with a hydraulic piston housing defining a fluid receiving chamber for telescopically accommodating the piston. One or more fluid passageways are provided to supply a fluid column to the hydraulic piston chamber sufficient to control piston displacement.
Quality shifts between gear ratios require precise and timely activation of an oncoming hydraulic clutch assembly and deactivation of another hydraulic clutch assembly. A quality shift in a gear drive assembly requires that total shift time between gear ratios is maintained at a minimum. By maintaining minimal shift times, the momentum imparted to the associated tractor or agricultural implement by one clutch carries over to the next clutch operation. Problems can result when the total shift time is excessive. When total shift time is excessive, tractor momentum is lost thus imparting more load to the oncoming clutch or cessation in tractor movement.
To effect rapid clutch engagement, a pressurized fluid flow column is delivered to the fluid receiving chamber. The fluid flow pressure is usually greater than that required to effect one hundred per cent of clutch torque capacity. The increased flow pressure is provided to very rapidly move the clutch actuator from a rest position to an engaged position. When the actuator arrives at its engaged position, however, there is a sudden deceleration of the actuator and the fluid column behind the actuator. The engagement of the actuator with the interleaved plates imparts a high-impact load to the clutch assembly resulting in a relatively high torque peak.
Although attractive because of their size and weight benefits, such clutch designs are not without problems. First, a relatively high torque peak results from the sudden impact of the clutch actuator with the clutch assembly. As such, a shearing force or action is imparted to the spline connection between the clutch plates and their respective rotational members. The repeated application of such shearing forces can, ultimately, destroy the spline connection between the clutch plates and an associated rotating member. Moreover, the sudden impact of the clutch actuator against the clutch assembly causes a relatively high impact load which is detrimental to the life of the clutch assembly. Furthermore, the sudden impact of the clutch actuator with the clutch assembly and the sudden deceleration of the fluid column creates a "water hammer" effect. Besides structural damage, such "water hammer" effect can generate an undesirable noise emanating from the transmission.
In view of the foregoing, the following criteria must be considered when attempting to find a solution to the above-captioned problems. Any purported solution to such problems cannot significantly effect the total shift time between gear ratios. Because of limited space constraints, any purported solution to such problems cannot occupy very much space. Moreover, any purported solution to the above-captioned problems must not significantly affect the torque capacity of the clutch.
In an effort to limit shift shocks incurred when the piston arrives at its engaging position with the clutch plates, it is known to use one Belleville spring positioned on that side of the clutch plates opposite to the clutch actuator. Such construction, however, undesirably requires linear displacement of the clutch plates on their splines attendant to cushioning. Moreover, the provision of one Belleville spring does not afford sufficient spring force necessary to solve the above-captioned problems.